Characterizing the greater-than-bulk melting behaviour of Ga-Al nanoalloys

Abstract

Phase diagrams are the most reliable way of predicting phase changes in a system. In this work, we create the phase diagram of the Ga(20-x)Al(x)+ bimetallic cluster system, for which the monometallic clusters display greater-than- bulk melting behaviour. Employing first-principles Born-Oppenheimer molecular dynamics in the microcanonical ensemble, we describe the solid-liquid-like phase transition in two distinct compositions Ga11Al9+ and Ga3Al17+. The clusters show peaks in specific heat at 824 K and 922 K respectively which is above the corresponding bulk alloy melting temperatures. Mean squared displacements, diffusion coefficients and atoms-in-molecules analysis emphasise the differences between the environments of the internal and surface atoms. An Arrhenius-like description of the 'ease' with which internal and surface sites (or atoms) can exchange positions with temperature is used to quantify the greater-than-bulk melting character which corroborates the view of a Ga(20-x)Al(x)+ cluster being a remnant of the corresponding bulk alloy material with additional characteristics specific to its size and composition.